May I suggest this device for finding plates in walls:

[Linked Image from stanleytools.com]

http://www.stanleytools.com/catalog_images/web_detail/55-099_web_detail.jpg

I have one of these makes short work of plaster, car windows, stubborn boards or anything that gets in the way of the 4-in-1 tools this beastie has. Mine cost AU$70 (around US$63), worth every cent.

More info from:
http://www.stanleytools.com/default...ubar%26%238482%3B+Functional+Utility+Bar